Chromium is an important constituent of a wide variety of alloys, including various steels as well as nickel-based, cobalt-based, molybdenum-based and copper-based alloys. Chromite ore from which the chrome metal is derived is found in abundance in relatively few places, notably in Southern Africa, whereas large amounts of chromium-containing scrap are generated in most industrial countries and recovery of the chromium therefrom is highly desirable. Other ores are equally available in relatively few places.
The recycling of chromium and other alloying metal containing scrap does not pose insurmountable difficulties in the case of stainless steel production. This is due in part to the ability to refine in the melting vessel as in the case of production of many complex alloys which may contain as many as five or even ten alloying elements. Such alloys are often very sensitive to contamination, and are produced by processes which do not lend themselves to any extensive refining in the melting vessel. As a result, producers of complex alloys, such as the so-called superalloys, tend to rely on pure metals for their feed, while superalloying metal-containing scrap is downgraded to produce less demanding alloys.
While procedures have been proposed in the past for separating the alloy constituents of scrap metals, none of the procedures to out knowledge has ever reached the stage of commercial implementation. This is undoubtedly ascribable to the cost and complexity of such procedures. Thus, methods of treating superalloy scrap are described in U.S. Pat. No. 3,544,309 (to A. W. Fletcher et al) as well as in the publication by P. T. Brooks et al entitled "Chemical Reclaiming of Superalloy Scrap", U.S. Dept. of Interior, Bureau of Mines, 1969. These methods are directed primarily at recovering nickel and cobalt from the scrap and entail a complete dissolution of the scrap, after which the various metals are separated by hydrometallurgical steps.
A process which avoids the slow and indiscriminate procedure of putting the whole of the scrap into solution is described in a published Japanese patent application, 73-44121 by T. Goto. The process described therein involves an initial pyrometallurgical treatment in which superalloy scrap is melted and blown with oxygen until such metals as aluminum, titanium, and silicon as well as much of the chromium content of the melt have been oxidized. Sulfur is then added and the bath is reblown to remove iron and chromium. The result is that iron and chromium are removed together in a slag high in titanium, aluminum and silicon, while a matte said to contain 60.8% Ni, 14.2% Co, 0.4% Cu and 24.5% S is derived after the blowing. The separate recovery of chromium or iron from the resulting slag is unlikely to be economically feasible.
Also worthy of mention in the present context is U.S. Pat. No. 3,313,601 (to O. F. Marvin) despite the fact that it does not relate to the treatment of scrap. The Marvin patent is concerned with the treatment of complex oxide ores, and describes an example wherein a chromite ore concentrate is heated to about 870.degree. C. in the presence of CS gas whereby it is said to be converted to a mixture of chromium sulfide, iron sulfide and iron oxide. The cooled mass is subsequently leached to leave a residue of chromium sulfide. The outcome of attemping such a solid state sulfidation on alloy scrap is unknown, and the subsequent hydrometallurgical separation process would be much more complex in a multi-component system.
The patent and general literature describing hydrometallurgical processes for the recovery of metals from solids in chloride solutions is extensive. The processes relate to recovery of metals, including substantially all the base metals, from a wide variety of matte sulphide concentrates and ores using different combinations of many leaching reagents including chlorine, hydrochloric acid, hydrochlorous acid, sodium hypochlorite, ferric chloride, cupric chloride, manganic chloride, sodium chloride, calcium chloride and other alkaline and alkaline-earth metal chlorides.
Few references, however, relate to the use of chlorine and a redox couple, such as cuprous-cupric chloride, to leach mattes, sulphide concentrates and alloys, particularly those containing metals selected from the group nickel, copper, and precious metals.
In U.S. Pat. No. 2,186,293 a process is described for recovery of nickel from matte by leaching in a solution containing cupric chloride and sufficient nickel chloride that the cuprous chloride formed upon dissolution of nickel remains in solution. The leach solution and residue are separated, nickel and copper are recovered from solution by electrolysis, and cupric chloride is regenerated for further leaching by reacting the solution remaining from the electrolysis with the chlorine produced thereby and returning the liquor to the leaching circuit.
A process for chlorine leaching of metals from sulphide ores is described in U.S. Pat. No. 1,943,337. The process is similar to that outlined above, but in this case the chlorine is fed to the same vessel in which the ore is treated thereby regenerating the leaching agent, such as ferric chloride, as it is consumed. As in the previous patent, however, no reference is made to preferential leaching of one metal from the solids with respect to other metals therein. The object of the process is clearly to leach as much metal value as possible from solids and there is no teaching of the possibility or desirability of preferential (or selective) leaching.
U.S. Pat. Nos. 2,829,966 and 2,835,569 relate to methods for recovery of gold from arseniureted ores containing nickel, cobalt and iron. Ore is leached in a mixture of HCl and chlorine to dissolve all the base metals, and the progress of the leach is followed by continuous monitoring of the redox potential. After dissolution of all the metals except gold, the continued addition of chlorine to the leach slurry causes an abrupt rise in potential until gold starts to dissolve, after which sufficient additional chlorine is fed to the slurry to dissolve most of the gold.
The lack of reference in these patents to either an interest in or means for separating nickel and chromium is consistent with the general literature on the leaching of sulphides by chlorides and chlorine. In fact, in U.S. Pat. No. 3,880,653 it is inferred that nickel could not be leached from chromium since chromium is less noble than nickel. In summary, the conclusion may be drawn that the prior art did not recognize that nickel can be selectively leached from chromium in the manner disclosed in the present invention.